We have connections to Sprint and Verio and I had BGP up and running with
Sprint for a couple of weeks. It appeared to me that they were advertising
our specific /24, 205.243.23.0, along with the rest of their advertisements.
Today, I turned up a BGP session with Verio, again just advertising our /24.
Soon afterward, with the help of lookingglass sites I noticed that
everywhere I checked was using the Verio path, including Sprint! I know
that a common cause of this would be if Sprint were not advertising our
more-specific /24, but Verio was. This would create a more specific match
through Verio. However, I don't think this is the case.
Next, I prepended our AS, 19105, twice on the Verio side and then checked
the LG sites again. Now, all were using the Sprint path. Next, I tried
reducing the number of prepends to one and again most places--including
Sprint--were using the Verio path.
What would cause this behavior? Even with no prepends, it looks like the AS
path for a lot of these locations (NAPs) was the same length. If so, and
both ISPs are advertising our /24 like they're supposed to be, why would I
have to prepend our AS twice to force routers to choose the Sprint path?
I think I just may have thought of a reason. If there were locations that
were aggregating Sprint's routes before advertising them to peers, that
would cause this behavior. But if that were the case, why would prepending
an AS matter at all? The Verio path would always be preferred because it is
most specific, right?
Arg.... I *thought* I understood this!
Any ideas?
Thanks,
John
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